Why do we let terrorists back in?

We have debated this before and a few have been denied entry back. This guy was a bomb maker in Afghanistan, luckily one of the few returnees that MI5 followed , caught red handed. There are too many to follow as he was and I cannot understand why people who have fought for the Taliban or ISIS are allowed back. Change the law if needed

Simple. We do not have enough border control officers for want of a better description, our entry ports are far too lax in security matters and other countries are happy to terrorists them move freely towards the UK as that gets them off their hands!